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C++ Attributes
source link: https://vorbrodt.blog/2019/04/03/c-attributes/
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C++ Attributes – Vorbrodt's C++ Blog
Compiler warnings; LLVM 8.0.0.
C++11 introduced standard attributes: a way to mark fragments of code with useful information for the developer or optimization information for the compiler. See a complete list of standard attributes here, Clang attributes here, and Microsoft attributes here. I will go over a few of them in this post.
- [[nodiscard]] – when specified with a function declaration, tells the compiler to emit a warning if function’s return value is ignored; when specified with a struct emits a warning wherever the struct is returned and ignored.
- [[fallthrough]] – suppresses compiler warning about a switch-case statement without a break; in other words, a case statement that falls through into another case.
- [[no_unique_address]] – tells the compiler to perform empty base optimization on marked data member.
- [[deprecated]] – emits a warning when marked function, struct, namespace, or variable is used.
- [[noreturn]] – tells the compiler that the marked function never returns; emits a warning if it does.
- [[maybe_unused]] – suppressed a warning when marked variable, function, or argument is not used.
- [[likely]] – tell the compiler this is the most likely path of execution; allows for optimizations.
Note: not all are supported on every compiler; I tested on LLVM 8.0.0 and GCC 8.2. Luckily the unsupported ones do not cause a compile error 
Below is an example code and a screenshot of compiler messages.
[[nodiscard]] int no_discard(int i)
switch(i)
case 1:
[[fallthrough]];
[[likely]] case 2: return i;
default: return i;
struct E {};
struct [[nodiscard]] S
int i;
[[no_unique_address]] E e;
S no_discard_struct()
return {};
struct [[deprecated("Use Q instead")]] P {};
[[deprecated("Use bar() instead")]] void foo() {}
[[noreturn]] void never_returns()
while(true);
int main()
[[maybe_unused]] int unused = 1;
no_discard(0);
no_discard_struct();
foo();
never_returns();
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